Thermoplastic, rigid foam materials, such as rigid, closed-cell, polyethylene foam materials, are employed, in sheet or board form, for a variety of uses. The foam material may vary in thickness, for example, from 2 to 6 inches for body boards or surf boards, and also may be employed at lower thicknesses; for example, 1/8th to 1/2 of an inch for foot treads. The polyethylene foam material may be heat-embossed, using a hot die, with various heat designs.
A polyethylene, foam sheet material also is commercially provided as a cross-linked, rigid, polyethylene, foam sheet material, wherein the polyethylene may be cross-linked, such as by irradiation or the use of organic cross-linking agents, such as organic peroxides, and wherein a foam material may be formed, such as by the employment of chemical blowing agents. Typically, a cross-linked, polyethylene foam sheet material is provided by admixing a selected amount of a blowing agent into polyethylene material in an extruder, so that cross-linking of the polyethylene, foam sheet material occurs on heating. Thereafter, the matrix material is conveyed through a long oven and expanded into a generally flat, cross-linked, polyethylene, foam bun or sheet material. However, in such a method of preparing cross-linked, polyethylene, foam sheet materials, the resulting foam sheet material is not characterized by good control of foam thickness, since the foam is expanded in a long, hot oven, so that the foam sheet product typically varies in thickness; for example, with a 1/2-inch-type sheet material, the thickness may vary by 5% up to 20%. In addition, the cross-linked, polyethylene, foam sheet materials or resulting products cannot be heat-embossed with a heated die, as with noncross-linked, polyethylene foam materials, particularly when the degree of cross-linking is higher, such as greater than 10%; for example, 40% to 90%.
It is desirable to provide for a new and unique method for preparing cross-linked, polyethylene, foam sheet products, such as a foam sheet material of controlled and accurate thickness, and to prepare cross-linked foam products having accurate, three-dimensional designs thereon, and to the product so produced.